The invention relates to a process for determining amounts of body fluids in a subject using bioelectrical response spectroscopy (BRS).
Determining amounts of body fluids in a subject and detecting changes in the level of fluids is an important clinical and research tool. For example, it is documented that athletes show decreases in plasma volume after they stop exercise training. Similarly, exposure to real or simulated micro gravity may decrease plasma or blood volume in astronauts. This decrease may affect the physical performance and safety of the astronauts during space flights. Being able to assess blood or plasma volumes during flight may help evaluate the effectiveness of measures designed to restore or maintain those volumes.
Prior art FIG. 1 shows a typical BRS method for determining volumes of body fluids in a subject. A signal generator 1 applies an alternating, low amperage signal through electrodes 2 to a subject 3. The alternating, low amperage signal travels through the subject 3 and is measured by an impedance analyzer 4 to determine a subject's impedance and resistance. The impedance and resistance are used to determine the subject's volumes of body fluids.
One known BRS method assumed that total body resistance was due to a total body water and was equivalent to the resistance of a wire. The volume of the wire is proportional to the resistance of the wire and directly related to the square of the length of the wire and the resistivity of the wire: EQU volume=.rho..multidot.length.sup.2 /R
where .rho. is the specific resistivity (ohms.multidot.cm) of the wire and R is the resistance. Using this electrical law and statistical regression, these investigators were able to develop estimation equations for total body water.
One known BRS method treated the body as a plurality of segmented conductors having uniform cross-sectional area. By measuring impedance to determine the composition of one or more body segments, a total body composition could be determined.
One known BRS method found that input signals of different frequencies would produce different resistances. These resistances were thought to be specific to different fluid compartments (e.g. extra-cellular fluid) and not to total body water.
One know BRS method used input signals of multiple frequencies to determine volumes of body fluids. These investigators used a Cole-Cole plot and iterative curve fitting techniques to determine extra-cellular and total body resistance.
One known BRS method assumed that capacitance was present in the body. This was based on a theory that cell membranes in the body acted like capacitors. Prior art FIG. 2 shows an electric circuit model of a human body that contains a capacitor. A series combination of resistor RI and capacitor C represented an intra-cellular impedance. A single resistor RE represented an extra-cellular resistance. The intra-cellular and extra-cellular branches were parallel to each other. A total impedance of the circuit between terminals 5 and 6 represented a total body impedance ZT that was thought to be due to a total body water.
One known BRS method assumed that inductance was also present in the body. This was based on a theory that vascular fluids in the body acted like an inductor. Prior art FIG. 3 shows a circuit model of the human body that included an inductor. A series combination of resistor RI and capacitor C represented an intra-cellular impedance. A series combination of resistor RE and inductor LE represented an extra-cellular impedance. The intra-cellular and extra-cellular branches were parallel to each other. A total impedance of the circuit between terminals 7 and 8 represented a total body impedance ZT. This model showed an 11% shift in blood volume after subjects completed 40 minutes of supine rest while resistance and estimates of total body water changed only 0.4-1.5%. These data suggested that the change in inductance, not resistance, may relate to known increase in blood volume (.sup..about. 10%) that occurs with supine rest.
Other known methods determined volumes of body fluids using dilution techniques that required injecting isotopic tracers into the subject's body. .sup.51 Cr-labeled hematocrit, .sup.125 I-labeled albumin, carbon monoxide, and inert dyes (e.g. Evans Blue) are examples of tracers used to assess blood, red cell, or plasma volumes. Most of these methods required multiple blood samples and sufficient time for the tracers to equilibrate within the vascular compartment. Also, repeat assessments must wait until the level of tracer in the blood decreased.